THE SEGMENTATION OF THE OVUM. 



87 



means of sections, there are some differences between the two types, and 

 several interesting features which deserve to be noticed in the segmentation 

 of the Elasmobranchii. In the first stages the furrows visible on the surface 

 are merely furrows, which do not meet so as to isolate distinct segments; they 

 merely, in fact, form a surface pattern. It is not till after the appearance 

 of the equatorial furrow that the segments begin to be distinctly isolated. 

 In the subsequent stages not only do the segments already existing in the 

 germinal disc increase by division, but fresh segments are continually 

 being formed from the adjacent yolk, and added to those already present 

 in the germinal disc. (Fig. 40.) This fact is one out of many which 



nx 



I - : 



rasi^Sr 

 ^fSf 



SPO" 



FIG. 46. SECTION THROUGH GERMINAL DISC OF A PRISTIURUS EMBRYO DURING THE 



SEGMENTATION. 



11. nucleus; iix. nucleus modified prior to division; nx '. modified nucleus of the 

 yolk; /. furrow appearing in the yolk adjacent to the germinal disc. 



prove that the germinal disc is merely part of the ovum characterized 

 by the presence of more protoplasm than the remainder which forms 

 the so-called food yolk. During the latest stages of segmentation there 

 appear in the yolk around the blastoderm a number of nuclei. (Fig. 46, 

 nx.) These are connected with a special protoplasmic network (already 

 described) which penetrates through the yolk. Towards the end of segmen- 

 tation, and during the early periods of development which succeed the 

 segmentation, these nuclei become very numerous. (Fig. 47 A, ri.) 

 Around many of them a protoplasmic investment is established, and 

 cells are thus formed which eventually enter the blastoderm. 



The result of segmentation is the formation of a lens-shaped mass 

 of cells lying in a depression on the yolk. In this a cavity appears, the 

 hoinologue of the segmentation cavity already spoken of. It lies at first in 

 the midst of the cells of the blastoderm, but very soon its floor of cells 

 vanishes, and it lies between the yolk and the blastoderm. (Fig. 47 A.) Its 

 subsequent history is given in a future Chapter. 



Segmentation proceeds in Osseous Fishes in nearly the same manner as in 

 Elasmobranchii. In some cases the germinal disc is small as compared with 

 the yolk, in other cases it is almost as large as the yolk. The only points 

 which deserve special notice are the folio wing: (1) Nuclei, precisely similar to 

 those in the Elasmobranch yolk, appear in the protoplasmic matter around 

 the germinal disc; (2) After the deposition of the ova there is present in some 

 iornis a network of protoplasm extending from the germinal disc through 



